this looks better:
firefox-hg $ time ./mach run
 0:00.83 /tmp/obj-x86_64-pc-linux-gnu/dist/bin/firefox -no-remote -profile /tmp/obj-x86_64-pc-linux-gnu/tmp/scratch_user
[24328] WARNING: NS_ENSURE_SUCCESS(rv, rv) failed with result 0x80004005: file /home/z/build/1packages/firefox-hg/makepkg_pacman/firefox-hg/src/firefox-hg/js/xpconnect/src/nsXPConnect.cpp, line 513
[24328] WARNING: NS_ENSURE_SUCCESS(rv, false) failed with result 0x80004005: file /home/z/build/1packages/firefox-hg/makepkg_pacman/firefox-hg/src/firefox-hg/dom/base/nsFrameMessageManager.cpp, line 1882
JavaScript error: file:///tmp/obj-x86_64-pc-linux-gnu/dist/bin/components/MainProcessSingleton.js, line 76: Error: No such property on self-hosted object: "String_static_normalize"
[24328] WARNING: NS_ENSURE_TRUE(global->Init()) failed: file /home/z/build/1packages/firefox-hg/makepkg_pacman/firefox-hg/src/firefox-hg/dom/base/nsFrameMessageManager.cpp, line 2203

Program /tmp/obj-x86_64-pc-linux-gnu/dist/bin/firefox (pid = 24328) received signal 11.
Stack:
#01: ???[/tmp/obj-x86_64-pc-linux-gnu/dist/bin/libxul.so +0x52c0179]
#02: ???[/usr/lib/libpthread.so.0 +0x10f00]
#03: ???[/tmp/obj-x86_64-pc-linux-gnu/dist/bin/libxul.so +0x1f29228]
#04: ???[/tmp/obj-x86_64-pc-linux-gnu/dist/bin/libxul.so +0x1f22b49]
#05: ???[/tmp/obj-x86_64-pc-linux-gnu/dist/bin/libxul.so +0x1f22b81]
#06: NS_InvokeByIndex[/tmp/obj-x86_64-pc-linux-gnu/dist/bin/libxul.so +0xbc8f39]
#07: ???[/tmp/obj-x86_64-pc-linux-gnu/dist/bin/libxul.so +0x17ab655]
#08: ???[/tmp/obj-x86_64-pc-linux-gnu/dist/bin/libxul.so +0x17b42c2]
#09: ???[/tmp/obj-x86_64-pc-linux-gnu/dist/bin/libxul.so +0x5614f81]
#10: ???[/tmp/obj-x86_64-pc-linux-gnu/dist/bin/libxul.so +0x560be92]
#11: ???[/tmp/obj-x86_64-pc-linux-gnu/dist/bin/libxul.so +0x5606e80]
#12: ???[/tmp/obj-x86_64-pc-linux-gnu/dist/bin/libxul.so +0x560bcbd]
#13: ???[/tmp/obj-x86_64-pc-linux-gnu/dist/bin/libxul.so +0x560bf98]
#14: ???[/tmp/obj-x86_64-pc-linux-gnu/dist/bin/libxul.so +0x560c361]
#15: ???[/tmp/obj-x86_64-pc-linux-gnu/dist/bin/libxul.so +0x5408137]
#16: ???[/tmp/obj-x86_64-pc-linux-gnu/dist/bin/libxul.so +0x17b21ec]
#17: ???[/tmp/obj-x86_64-pc-linux-gnu/dist/bin/libxul.so +0xbc9cc0]
#18: ???[/tmp/obj-x86_64-pc-linux-gnu/dist/bin/libxul.so +0xbc90fb]
Sleeping for 300 seconds.
Type 'gdb /tmp/obj-x86_64-pc-linux-gnu/dist/bin/firefox 24328' to attach your debugger to this thread.
^C
real	0m4.187s
user	0m0.804s
sys	0m0.951s


and installed(or maybe not - looks better):

$ fox
/tmp ~
[24621] WARNING: '!compMgr', file /home/z/build/1packages/firefox-hg/makepkg_pacman/firefox-hg/src/firefox-hg/xpcom/glue/nsComponentManagerUtils.cpp, line 63
[24621] WARNING: dependent window created without a parent: file /home/z/build/1packages/firefox-hg/makepkg_pacman/firefox-hg/src/firefox-hg/toolkit/components/startup/nsAppStartup.cpp, line 668
++DOCSHELL 0x7fb3f047b000 == 1 [pid = 24621] [id = 1]
++DOMWINDOW == 1 (0x7fb3f047d000) [pid = 24621] [serial = 1] [outer = (nil)]
++DOMWINDOW == 2 (0x7fb3f047e000) [pid = 24621] [serial = 2] [outer = 0x7fb3f047d000]
[24621] WARNING: NS_ENSURE_SUCCESS(rv, NS_ERROR_UNEXPECTED) failed with result 0x80004005: file /home/z/build/1packages/firefox-hg/makepkg_pacman/firefox-hg/src/firefox-hg/extensions/cookie/nsPermissionManager.cpp, line 852
JavaScript error: jar:file:///usr/lib/firefox/omni.ja!/components/XULStore.js, line 66: NS_ERROR_FAILURE: Component returned failure code: 0x80004005 (NS_ERROR_FAILURE) [nsIProperties.get]
[24621] WARNING: '!mLocalStore', file /home/z/build/1packages/firefox-hg/makepkg_pacman/firefox-hg/src/firefox-hg/dom/xul/XULDocument.cpp, line 2035
[24621] ###!!! ASSERTION: Double-initing a frame?: '!mContent', file /home/z/build/1packages/firefox-hg/makepkg_pacman/firefox-hg/src/firefox-hg/layout/generic/nsFrame.cpp, line 520
[24621] ###!!! ASSERTION: Double-initing a frame?: '!mContent', file /home/z/build/1packages/firefox-hg/makepkg_pacman/firefox-hg/src/firefox-hg/layout/generic/nsFrame.cpp, line 520
[24621] ###!!! ASSERTION: Double-initing a frame?: '!mContent', file /home/z/build/1packages/firefox-hg/makepkg_pacman/firefox-hg/src/firefox-hg/layout/generic/nsFrame.cpp, line 520

Program firefox (pid = 24621) received signal 11.
Stack:
#01: ???[/usr/lib/libpthread.so.0 +0x10f00]
#02: ???[/usr/lib/firefox/libxul.so +0x3f349d3]
#03: ???[/usr/lib/firefox/libxul.so +0x3ec2006]
#04: ???[/usr/lib/firefox/libxul.so +0x3eccf25]
#05: ???[/usr/lib/firefox/libxul.so +0x3efc453]
#06: ???[/usr/lib/firefox/libxul.so +0x3efcc25]
#07: ???[/usr/lib/firefox/libxul.so +0x3f1b3f6]
#08: ???[/usr/lib/firefox/libxul.so +0x3efae6c]
#09: ???[/usr/lib/firefox/libxul.so +0x3f06215]
#10: ???[/usr/lib/firefox/libxul.so +0x3f07247]
#11: ???[/usr/lib/firefox/libxul.so +0x3f8d223]
#12: ???[/usr/lib/firefox/libxul.so +0x3a1b6c7]
#13: ???[/usr/lib/firefox/libxul.so +0x3a205cf]
#14: ???[/usr/lib/firefox/libxul.so +0x3a291f1]
#15: ???[/usr/lib/firefox/libxul.so +0x3a2c1a0]
#16: ???[/usr/lib/firefox/libxul.so +0x3a18493]
#17: ???[/usr/lib/firefox/libxul.so +0xbac944]
#18: ???[/usr/lib/firefox/libxul.so +0xbf743b]
#19: ???[/usr/lib/firefox/libxul.so +0x45e7a85]
#20: ???[/usr/lib/firefox/libxul.so +0x457d83a]
#21: ???[/usr/lib/firefox/libxul.so +0x457e243]
#22: ???[/usr/lib/firefox/libxul.so +0xa7dee8]
#23: ???[/usr/lib/firefox/libxul.so +0x491e85e]
#24: ???[/usr/lib/firefox/libxul.so +0x491fbc0]
#25: ???[/usr/lib/firefox/libxul.so +0x492322b]
#26: XRE_main[/usr/lib/firefox/libxul.so +0x492362f]
#27: ???[firefox +0x6149]
#28: ???[firefox +0x5713]
#29: __libc_start_main[/usr/lib/libc.so.6 +0x20741]
#30: _start[firefox +0x5989]
#31: ??? (???:???)
Sleeping for 300 seconds.
Type 'gdb firefox 24621' to attach your debugger to this thread.
^C

